An industrial customer called his local utility representative to report nuisance trips of his ground fault interrupter (GFI) on his main panel. The customer initially had his GFI set for trip at 200 amps with no time delay. To reduce the nuisance trips, the customer had decreased the GFI sensitivity to 400 amp trip with 18 cycle (0.3 second) delay. He had no GFI trips at this level but requested assistance in locating the cause for earlier nuisance shutdowns.

GFIs are used at the main panel of many industrial and commercial systems to protect against ground faults. They should not be confused with ground fault circuit interrupters which are often used on 120 volt circuits.

GFIs usually protect for ground faults using one of two methods. One method is to monitor all three phase conductors and neutral to detect ground fault current. Another is to monitor the neutral-to-ground bond for ground current surges at the customer main panel. Monitoring the neutral-to-ground bond can make the GFI susceptible to neutral current surges.

FINDINGS

Since this customer has a GFI that watches current in the neutral-to-ground bond, a disturbance analyzer was used to monitor the bond. Steady-state current in the neutral-to-ground bond was low. However, the disturbance analyzer recorded a 5 millisecond (1/3 cycle) current surge when switching a nearby 12.47 kV capacitor bank. Figure 1 shows the current in the neutral-to-ground bond as each phase of the capacitor energized.

Figure 1: Current Surge in Neutral-Ground Bond

Consider Figure 2, a simplified model of the system. Random closing of the capacitor switches causes a transient current imbalance and surge in neutral current. Some of the surge current will flow through the service transformer and the customer main GFI.

Figure 2: Surge Current Path Thru GFI

SOLUTION

Adding at least three cycles time delay should be sufficient to eliminate nuisance trips on GFIs that monitor current in the neutral-to-ground bond. In this case, moving the GFI setting back to 200 amp trip and 6 cycles delay (the next shortest delay) improved ground fault sensitivity without suffering nuisance trips.

The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.